This invention relates broadly to methods for manufacturing semi-conductor devices. More particularly, it relates to a continuous controlled differential atmosphere system to be employed in the fabrication of such devices. In particular, the invention relates to the implantation of impurity ions in the manufacture of solid state devices and micro circuits.
A most critical phase of the manufacture of semi-conductor devices and one which has been the subject of intensive investigation and development is the process of doping. In this procedure, the crystal structure of pre-designed areas or zones of the semi-conductor element is infused to a pre-determined level with "impurities", i.e., atoms of different chemical structure, in order to impart to that part of the semi-conductor element a particular desired conductivity. Thus, a semi-conductor substance such as N-type silicon may be doped with atoms of an acceptor impurity such as boron to convert a region in the silicon element from N to P-type conductivity. Conversely, P-type silicon may be doped with atoms such as phosphorus to convert a region to N-type conductivity.
Prior art techniques for doping include diffusion, evaporation, epitaxial growth and sputtering. More recently, techniques have been developed for bombarding the semi-conductor with a beam of impurity ions generated by a conventional high voltage accelerator. In this procedure the ions, which have been accelerated to a high velocity, penetrate the surface of the semi-conductor material and thus may be said to be "implanted" in the semi-conductor. Under controlled conditions the implated ions can be made to assume lattice positions within the semi-conductor, thus enabling them to act as donors or acceptors.
Unlike other techniques, ion bombardment is not primarily thermally activated. Thus reactions with the semi-conductor surface and/or masking materials, alteration of junction profiles within the semi-conductor and uneven distribution of the doping impurity within the doped area, all of which are inherent byproducts of other doping techniques, may be avoided to a large extent by the use of ion bombardment. In theory then, significant advantages over the more traditional junction formation techniques are available with ion implantation.
A significant deterrent, however, to the utilization of ion implantation doping techniques in large scale semi-conductor manufacturing has been the inability to reduce processing time and lower processing costs, particularly by automation. The prime factor here is the requirement that the ion bombarding technique be carried out in a vacuum. An example of a present day commercial technique involves placing a batch of semi-conductor elements in a large chamber, evacuating the chamber and employing mechanical, robot-like arms to pick up the semi-conductor elements individually, place them in appropriate position for the ion beam and remove them at the appropriate time. When the complete batch is implanted, the vacuum must be broken to remove the implanted wafers and insert a new batch. Quite naturally, the larger the vacuum chamber the more difficult and expensive it is to attain and maintain appropriate evacuated conditions in repeated fashion. This procedure is thus not only bulky and cumbersome but also is accompanied by substantial cost and lower reliability. Up to the time of the present invention, the requirement that implantation be carried out in an evacuated chamber appeared to be a major obstacle in the development of a reasonable cost, automatic ion implantation procedure.